## Bar Magnet as an Equivalent Solenoid – Class 12 | Chapter – 5 | Physics Short Notes Series PDF for NEET & JEE

Bar Magnet as an Equivalent Solenoid: A bar magnet can be considered as an equivalent solenoid, as both produce a magnetic field in a similar way. A solenoid is a coil of wire that carries an electric current, and when a current is passed through a solenoid, it produces a magnetic field. Similarly, a bar magnet also produces a magnetic field due to the alignment of its magnetic dipoles.

## Bar Magnet as an Equivalent Solenoid

When a bar magnet is considered as an equivalent solenoid, the following similarities can be observed:

• Both produce a magnetic field that is strongest at the poles.
• The direction of the magnetic field is the same at all points along the length of the solenoid or magnet.
• The magnetic field inside the solenoid or magnet is uniform.
• Both have a north and south pole.
• Both can be used to attract or repel magnetic materials.

## Derivation of Bar Magnet as an Equivalent Solenoid

The magnetic field produced by a solenoid can be calculated using the following equation:

B = μ₀ * n * I

where B is the magnetic field, μ₀ is the permeability of free space, n is the number of turns per unit length, and I is the current flowing through the solenoid.

A bar magnet can be considered as an equivalent solenoid with a certain number of turns per unit length, and a current flowing in each turn. The magnetic field produced by a bar magnet is given by the following equation:

B = (μ₀/4π) * (2M/r3)

where B is the magnetic field, μ₀ is the permeability of free space, M is the magnetic moment of the bar magnet, r is the distance from the center of the bar magnet, and π is the mathematical constant pi.

To derive the equivalent solenoid model of a bar magnet, we need to find the number of turns per unit length, and the current flowing in each turn. This can be done by equating the above two equations:

(μ₀/4π) * (2M/r3) = μ₀ * n * I

Rearranging this equation, we get:

n = (2M/πr2) * (1/I)

Here,

• (2M/πr2) represents the area of the cross-section of the bar magnet,
• (1/I) represents the number of turns per unit length.

From this equation, we can see that the number of turns per unit length and the current flowing in each turn are inversely proportional to each other. This means that as the current increases, the number of turns per unit length decreases, and vice versa.

Using this relationship, we can model a bar magnet as an equivalent solenoid with a certain number of turns per unit length and a current flowing in each turn, which produces a magnetic field that is similar to that of the bar magnet. This equivalent solenoid model can be used to analyze the behavior of bar magnets and to predict their interaction with other magnetic materials.

## Difference between a Bar Magnet and a Solenoid

A bar magnet and a solenoid are two different types of magnetic structures, with some important differences:

• Source of magnetic field: A bar magnet produces a magnetic field due to the alignment of its magnetic dipoles, while a solenoid produces a magnetic field due to the flow of electric current through its coil.
• Shape and size: A bar magnet is a solid object with a definite shape and size, while a solenoid is typically a coil of wire with a cylindrical shape, and its size can be adjusted by changing the number of turns in the coil or the diameter of the coil.
• Field strength: The magnetic field of a bar magnet is constant, and its strength depends on the magnetic moment of the magnet, while the magnetic field of a solenoid depends on the number of turns per unit length, the current flowing through the coil, and the permeability of the material in the core of the coil.
• Magnetic polarity: A bar magnet has a north pole and a south pole, while a solenoid can have a magnetic field with only one polarity or it can be reversed by changing the direction of the current flow.
• Applications: Bar magnets are commonly used in a wide range of applications, such as in electric motors, generators, compasses, and speakers, while solenoids are used in a variety of devices, such as electromagnets, relays, and doorbells.

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